Am I missing something? There is no lesson learned here. No offer of help was accepted and nothing went wrong. Why beware? He hasn't even tried it. Hell I can't even really parse if there ever was an offer to help, just some suggestions.
From the very limited information presented, it's not clear to me why the author would judge this offer to be a selfish act, or why this anecdote would support his conclusion to "beware offers of help".
My explanation is they did not have what is needed like tech skills or they were after taking over existing users.
Idea of scaling something up as author noted usually comes with idea that money will eventually follow. Post author is not money driven so it might be that person offering was also different in that way.
I've encountered this with dozens of contributors myself.